Thyroid hormones are critical for normal growth and development and for maintaining metabolic homeostasis (Paul M. Yen Physiological Review, Vol. 81(3): pp. 1097-1126 (2001)). Circulating levels of thyroid hormones are tightly regulated by feedback mechanisms in the hypothalamus/pituitary/thyroid (HPT) axis. Thyroid dysfunction leading to hypothyroidism or hyperthyroidism clearly demonstrates that thyroid hormones exert profound effects on cardiac function, body weight, metabolism, metabolic rate, body temperature, cholesterol, bone, muscle and behavior.
Thyroid hormone is produced by the thyroid gland and secreted into circulation as two distinct forms, 3,5,3′,5′-tetra-iodo-L-thyronine (T4) and 3,5,3′-tri-iodo-L-thyronine (T3). While T4 is the predominant form secreted by the thyroid gland, T3 is the more biologically active form. T4 is converted to T3 by tissue specific deiodinases in all tissues but predominantly in the liver and kidney. The biological activity of thyroid hormones is mediated by thyroid hormone receptors (TRs) (M. A. Lazar Endocrine Reviews, Vol. 14: pp. 348-399 (1993)). TRs belong to the superfamily known as nuclear receptors. TRs form heterodimers with the retinoid receptor that act as ligand-inducible transcription factors. TRs have a ligand binding domain, a DNA binding domain, and an amino terminal domain, and regulate gene expression through interactions with DNA response elements and with various nuclear co-activators and co-repressors. The thyroid hormone receptors are derived from two separate genes, α and β. These distinct gene products produce multiple forms of their respective receptors through differential RNA processing. The major thyroid receptor isoforms are α1, α2, β1 and β2. Thyroid hormone receptors α1, β1 and β2 bind thyroid hormone. It has been shown that the thyroid hormone receptor subtypes can differ in their contribution to particular biological responses. Recent studies suggest that TRβ1 plays an important role in regulating TRH (thyrotropin releasing hormone) and on regulating thyroid hormone actions in the liver. TRβ2 plays an important role in the regulation of TSH (thyroid stimulating hormone) (Abel et. al. J. Clin. Invest., Vol 104: pp. 291-300 (1999)). TRβ1 plays an important role in regulating heart rate (B. Gloss et. al. Endocrinology, Vol. 142: pp. 544-550 (2001); C. Johansson et. al. Am. J. Physiol., Vol. 275: pp. R640-R646 (1998)).
Some of the effects of thyroid hormones may be therapeutically beneficial if adverse effects can be minimized or eliminated (Paul M. Yen Physiological Reviews, Vol. 81(3): pp. 1097-1126 (2001); Paul Webb Expert Opin. Investig. Drugs, Vol. 13(5): pp. 489-500 (2004)). For example, thyroid hormones increase metabolic rate, oxygen consumption and heat production and thereby reduce body weight. Reducing body weight will have a beneficial effect in obese patients by ameliorating the co-morbidities associated with obesity, and may also have a beneficial effect on glycemic control in obese patients with Type 2 diabetes.
Another therapeutically beneficial effect of thyroid hormone is the lowering of serum low density lipoprotein (LDL) (Eugene Morkin et. al. Journal of Molecular and Cellular Cardiology, Vol. 37: pp. 1137-1146 (2004)). It has been found that hyperthyroidism is associated with low total serum cholesterol, which is attributed to thyroid hormone increasing hepatic LDL receptor expression and stimulating the metabolism of cholesterol to bile acids (J. J. Abrams et. al. J. Lipid Res., Vol. 22: pp. 323-38 (1981)). Hypothyroidism, in turn, has been associated with hypercholesterolemia and thyroid hormone replacement therapy is known to lower total cholesterol (M. Aviram et. al. Clin. Biochem., Vol. 15: pp. 62-66 (1982); J. J. Abrams et. al. J. Lipid Res., Vol. 22: pp. 323-38 (1981)). Thyroid hormone has been shown in animal models to have the beneficial effect of increasing HDL cholesterol and improving the ratio LDL to HDL by increasing the expression of apo A-1, one of the major apolipoproteins of HDL (Gene C. Ness et. al. Biochemical Pharmacology, Vol. 56: pp. 121-129 (1998); G. J. Grover et. al. Endocrinology, Vol. 145: pp. 1656-1661 (2004); G. J. Grover et. al. Proc. Natl. Acad. Sci. USA, Vol. 100: pp. 10067-10072 (2003)). Through its effects on LDL and HDL cholesterol, it is possible that thyroid hormones may also lower the risk of atherosclerosis and other cardiovascular diseases. The incidence of atherosclerotic vascular disease is directly related to the level of LDL cholesterol. Additionally, there is evidence that thyroid hormones lower Lipoprotein (a), an important risk factor which is elevated in patients with atherosclerosis (Paul Webb Expert Opin. Investig. Drugs, Vol. 13(5): pp. 489-500 (2004); de Bruin et. al. J. Clin. Endo. Metab., Vol. 76: pp. 121-126 (1993)).
With the incidence of obesity and its co-morbidities, diabetes, metabolic syndrome, and atherosclerotic vascular disease rising at epidemic rates, the utility of compounds capable of treating these diseases would be highly desirable. To date, the therapeutic uses of the naturally occurring thyroid hormone have been limited by the adverse side effects associated with hyperthyroidism, especially cardiovascular toxicity.
Therefore, efforts have been made to synthesize thyroid hormone analogs which exhibit increased thyroid hormone receptor beta selectivity and/or tissue selective action. Such thyroid hormone mimetics may yield desirable reductions in body weight, lipids, cholesterol, and lipoproteins, with reduced impact on cardiovascular function or normal function of the hypothalamus/pituitary/thyroid axis (A. H. Underwood et al. Nature, Vol. 324: pp. 425-429 (1986), G. J. Grover et. al. PNAS, Vol. 100: pp. 10067-10072 (2003); G. J. Grover Endocrinology, Vol. 145: pp. 1656-1661(2004); Yi-lin Li et. al. PCT Int. Appl. WO 9900353 (1999); Thomas S. Scanlan et. al. PCT Int. Appl. WO 9857919 (1998); Keith A. Walker et. al. U.S. Pat. No. 5,284,971 (1994); Mark D. Erion et. al. PCT Int. Appl. WO 2005051298 (2005); Malm Johan Current Pharmaceutical Design, Vol. 10(28): pp. 3525-3532 (2004); Expert Opin. Ther. Patents, Vol. 14: pp. 1169-1183 (2004); Thomas S. Scalan Current Opinion in Drug Discovery & Development, Vol. 4 (5): pp. 614-622 (2001); Paul Webb Expert Opinion on Investigational Drugs, Vol. 13 (5): pp. 489-500 (2004)).
The development of thyroid hormone analogs which avoid the undesirable effects of hyperthyroidism and hypothyroidism while maintaining the beneficial effects of thyroid hormones would open new avenues of treatment for patients with metabolic diseases such as obesity, hyperlipidemia, hypercholesterolemia, diabetes and other disorders and diseases such as liver steatosis and NASH, atherosclerosis, cardiovascular diseases, hypothyroidism, thyroid cancer, thyroid diseases, and related disorders and diseases.
Pyridazinone compounds that are structurally different from the compounds of the present invention have been previously disclosed (Teruomi et. al. Agricultural and Biological Chemistry, Vol. 38(6): pp. 1169-76 (1974); P. D. Leeson et. al. J. Med. Chem. Vol. 32: pp. 320-326 (1989); Eur. Pat. Appl. EP 188351 (1986); Damien John Dunnington PCT Int. Appl. WO 9702023 (1997); and Eur. Pat. Appl. EP 728482 (1996)).
There is still a need, however, for novel thyroid hormone prodrugs such as, for example, novel pyridazinone prodrugs, that have the beneficial effects of thyroid hormone while avoiding the undesirable effects.